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PubMed2006

Apicomplexa genes involved in the host cell invasion: the Cpa135 protein family.

The availability of a bulk of genomic data of Apicomplexa parasites is a unique opportunity to identify groups of related proteins that are characteristic of this phylum. The Cpa135 protein of Cryptosporidium parvum is expressed and secreted through the apical complex at the invasive stage of sporozoite. This protein is characterised by an LCCL domain, a common trait of various secreted proteins within Apicomplexa. Using the Cpa135 as a "virtual template", we have identified Cpa135 orthologous genes in four apicomplexan species (Plasmodium falciparum, Theileria parva, Toxoplasma gondii and Eimeria tenella). In addition, the architecture of the deduced proteins shows that the Cpa135-related proteins are a distinct family among the apicomplexan LCCL proteins.

Animals↗
PubMed1979

Rhabdospora thelohani Laguessé, 1895 (Apicomplexa): new host and geographic records with taxonomic considerations.

New fish species and geographic records for Rhabdospora thelohani Laguessé, 1895 (rodlet cells) are presented. Additionally, the ultrastructure of R. thelohani in Alburnoides bipunctatus ohridanus Karaman, Borostomias antarcticus (Lönnberg), Leuciscus cephalus albus Bonaparte and Rutilus rubilio (Bonaparte) is compared with that reported by other authors and with members of Subphylum Apicomplexa. The ultrastructure of R. thelohani was similar in all the fish species examined; however, the organism was not present in all members of any single species and had intertissue density variations. Rhabdospora thelohani is pyriform, averaging in size 7 X 12 micrometer, with a basal nucleus. The surface complex is composed of a layer (0.5 micrometer diameter) formed by microfilaments (9.3 nm) and an outer trilaminar membrane (9.3 nm). The cytoplasm contains structures identical to rhoptries, micronemes and subpellicular microtubules. Mitochondria, Golgi apparatus, and rough endoplasmic reticulum were not observed, althouth free ribosomes were present and arranged in a vesicular pattern. The observations suggest that the organism moves between cell of epithelial layers and is either released into a lumen intact or passively or actively discharges its contents into a lumen. Results from this study indicate that R. thelohani should be considered a member of Apicomplexa unless definitive evidence is presented to the contrary.

Animals↗
PubMed2005

Dynamic organization of microtubules and microtubule-organizing centers during the sexual phase of a parasitic protozoan, Lecudina tuzetae (Gregarine, Apicomplexa).

Lecudina tuzetae is a parasitic protozoan (Gregarine, Apicomplexa) living in the intestine of a marine polychaete annelid, Nereis diversicolor. Using electron and fluorescence microscopy, we have characterized the dynamic changes in microtubule organization during the sexual phase of the life cycle. The gametocyst excreted from the host worm into seawater consists of two (one male and one female) gamonts in which cortical microtubule arrays are discernible. Each gamont undergoes multiple nuclear divisions without cytokinesis, resulting in the formation of large multinucleate haploid cells. After cellularization, approximately 1000 individual gametes are produced from each gamont within 24 h. Female gametes are spherical and contain interphase cytoplasmic microtubule arrays emanating from a gamma-tubulin-containing site. In male gametes, both interphase microtubules and a flagellum with "6 + 0" axonemal microtubules extend from the same microtubule-organizing site. At the beginning of spore formation, each zygote secretes a wall to form a sporocyst. Following meiotic and mitotic divisions, each sporocyst gives rise to eight haploid cells that ultimately differentiate into sporozoites. The ovoid shaped sporocyst is asymmetric and forms at least two distinctive microtubule arrays: spindle microtubules and microtubule bundles originating from the protruding apical end corresponding to the dehiscence pole of the sporocyst. Because antibodies raised against mammalian centrosome components, such as gamma-tubulin, pericentrin, Cep135, and mitosis-specific phosphoproteins, react strongly with the microtubule-nucleating sites of Lecudina, this protozoan is likely to share common centrosomal antigens with higher eukaryotes.

Animals↗
PubMed2006

Multi-membrane-bound structures of Apicomplexa: II. the ovoid mitochondrial cytoplasmic (OMC) complex of Toxoplasma gondii tachyzoites.

Apicomplexa including the causative agents of toxoplasmosis and malaria reportedly possess one or few tubular-shaped mitochondria that permeate, more or less branched, throughout these unicellular parasites. Electron micrographs generated herein from serial-sectioned Toxoplasma gondii tachyzoites demonstrated, however, a greater diversity regarding both the shape of the cultured parasite's single mitochondrion and its sub-structural organization. Moreover, a unique subcellular construction was detected that basically comprised a pouch-shaped subdivision of the tachyzoite mitochondrion plus a fraction of parasitic cytoplasm enclosed therein. This composite assembling, termed ovoid mitochondrial cytoplasmic (OMC) complex, characteristically displayed a highly reduced matrix lumen of its mitochondrial border construction, which furthermore often failed to possess any cristae or contained tightly pleated cristae, thus creating a pouch-shaped multi-laminar wall of four or more membranous layers, respectively. Given this architecture, cross-sectioned OMC complexes of T. gondii tachyzoites frequently mimicked in size and shape the parasites' plastid-like organelle (apicoplast). Moreover, like the apicoplast, the OMC complex was often found adjacent to the tachyzoite's single Golgi complex and constantly located in close proximity to the outer membrane of the parasite's nuclear envelope. The T. gondii OMC complex differed, however, from the apicoplast in its exact fine structural organization and a stage-restricted presence that was apparently linked to mitochondrial growth and/or division. Any special function(s) possibly performed by the T. gondii OMC complex remains, nevertheless, to be elucidated.

Animals↗
PubMed1993

Analysis of dinucleotide frequency and codon usage in the phylum Apicomplexa.

Dinucleotide frequency (DiF) and codon usage (cu) were analysed in gene sequences from four parasitic protozoa, Babesia bovis, Theileria parva, Toxoplasma gondii and Eimeria tenella, of the phylum Apicomplexa. In keeping with the 'genome hypothesis', cu was found to be non-random and species specific in these organisms, although cu among members of the same subclass was found to be very similar. Several low-usage (lu) codons were identified, and the usage of lu codons appears to be related to the taxonomic position of the organisms under study. A comparison of the observed/expected DiF ratios obtained from gene coding regions revealed a low frequency of the TA and CG dinucleotides in all organisms studied. A comparison of these DiF ratios with those found in rRNA-encoding genes and in introns, showed that in the parasites, B. bovis and Th. parva (representing the piroplasms), the low frequency of dinucleotides appeared to be the result of coding pressure alone. In T. gondii and E. tenella (representing the coccidia), however, coding pressure could not completely explain differences in DiF.

Animals↗
PubMed2004

Role of proteases in host cell invasion by Toxoplasma gondii and other Apicomplexa.

The process of invasion by apicomplexan parasites is a carefully coordinated process involving the regulated release of specialized secretory organelles. Several lines of evidence suggest that proteases are critical for the assembly and trafficking of organellar content proteins. Further, invasion is accompanied by cleavage and shedding of secreted proteins as host cell invasion occurs. Recent studies in Toxoplasma gondii and other Apicomplexa have led to the identification of proteases that may mediate these processing events. Among these are subtilases, subtilisin-like serine proteinases that have essential roles in processing of secreted proteins in prokaryotes and eukaryotes. Other studies suggest that cysteine proteinases or rhomboid proteases, a newly described class of serine proteinases, may be important. In addition to providing insights into the invasion process, characterization of invasion proteases may lead to identification of novel targets for antiparasitic chemotherapy.

Amino Acid Sequence↗
PubMed1996

The growing importance of the plastid-like DNAs of the Apicomplexa.

Organisms in the phylum Apicomplexa possess, in addition to their mitochondrial genome, an extrachromosomal DNA that possesses significant similarities with the extrachromosomal genomes of plastids. To date, the majority of data on these plastid-like DNAs have been obtained from the human malarial organism, Plasmodium falciparum. In common with plastid DNAs, the plastid-like DNA of P. falciparum possesses genes for DNA-dependent RNA polymerase subunits beta and beta 1 and for organellar-like large- and small-subunits ribosomal RNAs. Both the polymerase subunit and ribosomal RNA gene sequences share a number of features with those from plastid DNAs. In addition, the ribosomal RNA genes are organised in an inverted repeat arrangement, reminiscent of plastid DNAs. Additional molecular features shared between the 2 genomes are discussed. Plastid-like DNAs have also been identified in other Plasmodium species as well as Toxoplasma gondii, Eimeria tenella, Babesia bovis and a number of Sarcocystis species. A cryptic organelle often observed in apicomplexans has been proposed as the organelle that harbours the plastid-like DNAs, but conclusive evidence for this has not yet been obtained. Although approximately 1/2 of the plastid-like DNA of P. falciparum has been sequenced to date, no function has yet been ascribed to this DNA or its putative organelle. Phylogenetic inferences based on sequence data from this DNA have indicated an evolutionary origin from photosynthetic organisms, but the true provenance of the plastid-like DNAs remains to be determined. Because of the specific nature of the plastid-like DNAs, they may prove useful as effective targets for chemotherapeutics.

Animals↗
PubMed1998

Plastids are widespread and ancient in parasites of the phylum Apicomplexa.

Current evidence supports the presence of a non-photosynthetic chloroplast-like organelle in several apicomplexan parasites, including Plasmodium falciparum and Toxoplasma gondii. This apicomplexan organelle, referred to here as the "plastid", may have been acquired through a primary or secondary endosymbiosis of a photosynthetic organism. Alternatively, apicomplexan plastids may have been acquired through several independent endosymbiotic events, as appears to be the case for the acquisition of chloroplasts by dinoflagellates. The likelihood of multiple origins of an apicomplexan plastid is enhanced by the close evolutionary relatedness of apicomplexan and dinoflagellate taxa. In this study, we have tested the hypothesis that apicomplexan plastids are derived from a single ancient ancestor. Two lines of evidence supporting this hypothesis are presented. First, this study supports the widespread presence of plastid DNA in apicomplexan species. Second, the topologies of the phylogenetic trees derived from plastid and nuclear-encoded rRNA gene sequences suggest the co-evolution of the DNAs localised in these two compartments. Taken together, these data support a single ancient lineage for the plastids of parasites in the phylum Apicomplexa.

Animals↗
PubMed2002

Sexual differentiation and sex determination in the Apicomplexa.

Protozoan parasites of the phylum Apicomplexa have complex life cycles involving various types of asexual division that allow rapid proliferation of parasites within one or more hosts. Such replication is punctuated by obligate sexual differentiation that produces male and female gametocytes. These stages are transmissible to haematophagous vectors or are necessary ultimately to form resistant cysts that are released into the environment. This article examines the sexual differentiation of apicomplexan parasites as it relates to the timing of commitment and the mechanism of the switch from asexual proliferation to the development of male and female sexual stages.

Animals↗
PubMed2006

Comparative genomic and phylogenetic analyses of calcium ATPases and calcium-regulated proteins in the apicomplexa.

The phylum Apicomplexa comprises a large group of early branching eukaryotes that includes a number of human and animal parasites. Calcium controls a number of vital processes in apicomplexans including protein secretion, motility, and differentiation. Despite the importance of calcium as a second messenger, very little is known about the systems that control homeostasis or that regulate calcium signaling in parasites. The recent completion of many apicomplexan genomes provides new opportunity to define calcium response pathways in this group of parasites in comparison to model organisms. Whole-genome comparison between the apicomplexans Plasmodium spp., Cryptosporidium spp., and Toxoplasma gondii revealed the presence of several P-Type Ca2+ transporting ATPases including a single endoplasmic reticulum (ER)-type sarcoplasmic-endoplasmic reticulum Ca2+ ATPase, several Golgi-like Ca2+ ATPases, and a single Ca2+/H+ exchanger. Only T. gondii showed evidence of plasma membrane-type Ca2+ ATPases or voltage-gated calcium channels. Despite pharmacological evidence for IP3 and ryanodine-mediated calcium release, animal-type calcium channels were not readily identified in parasites, indicating they are more similar to plants. Downstream of calcium release, a variety of EF-hand-containing proteins regulate calcium responses. Our analyses detected a single conserved calmodulin (CaM) homologue, 3 distinct centrin (CETN)-caltractin-like proteins, one of which is shared with ciliates, and a variety of deep-branching, CaM-CETN-like proteins. Apicomplexans were also found to contain a wide array of calcium-dependent protein kinases (CDPKs), which are commonly found in plants. Toxoplasma gondii contains more than 20 CDPK or CDPK-related kinases, which likely regulate a variety of responses including secretion, motility, and differentiation. Genomic and phylogenetic comparisons revealed that apicomplexans contain a variety of unusual calcium response pathways that are distinct from those seen in vertebrates. Notably, plant-like pathways for calcium release channels and calcium-dependent kinases are found in apicomplexans. The experimental flexibility of T. gondii should allow direct experimental manipulation of these pathways to validate their biological roles. The central importance of calcium in signaling and development, and the novel characteristics of many of these systems, indicates that parasite calcium pathways may be exploited as new therapeutic targets for intervention.

Amino Acid Sequence↗
PubMed2004

Comparative analysis of apicomplexa and genomic diversity in eukaryotes.

The apicomplexans Plasmodium and Cryptosporidium have developed distinctive adaptations via lineage-specific gene loss and gene innovation in the process of diverging from a common parasitic ancestor. The two lineages have acquired distinct but overlapping sets of surface protein adhesion domains typical of animal proteins, but in no case do they share multidomain architectures identical to animals. Cryptosporidium, but not Plasmodium, possesses an animal-type O-linked glycosylation pathway, along with >30 predicted surface proteins having mucin-like segments. The two parasites have notable qualitative differences in conserved protein architectures associated with chromatin dynamics and transcription. Cryptosporidium shows considerable reduction in the number of introns and a concomitant loss of spliceosomal machinery components. We also describe additional molecular characteristics distinguishing Apicomplexa from other eukaryotes for which complete genome sequences are available.

Animals↗
PubMed1998

Gene discovery by EST sequencing in Toxoplasma gondii reveals sequences restricted to the Apicomplexa.

To accelerate gene discovery and facilitate genetic mapping in the protozoan parasite Toxoplasma gondii, we have generated >7000 new ESTs from the 5' ends of randomly selected tachyzoite cDNAs. Comparison of the ESTs with the existing gene databases identified possible functions for more than 500 new T. gondii genes by virtue of sequence motifs shared with conserved protein families, including factors involved in transcription, translation, protein secretion, signal transduction, cytoskeleton organization, and metabolism. Despite this success in identifying new genes, more than 50% of the ESTs correspond to genes of unknown function, reflecting the divergent evolutionary status of this parasite. A newly recognized class of genes was identified based on its similarity to sequences known only from other members of the same phylum, therefore identifying sequences that are apparently restricted to the Apicomplexa. Such genes may underlie pathways common to this group of medically important parasites, therefore identifying potential targets for intervention.

Animals↗
PubMedDate not supplied

Ascogregarina saraviae n. sp. (Apicomplexa: Lecudinidae) in Lutzomyia lichyi (Diptera: Psychodidae).

Ascogregarina saraviae n. sp. (Apicomplexa: Lecudinidae) is described from wild-caught Lutzomyia lichyi (Diptera: Psychodidae) females. Gametocysts adhered to the hemocoel side of the genital accessory gland walls and oocysts were injected into their lumina. Sporulated oocysts were ellipsoidal, 12.4 x 5.8 (11.6-13.1 x 5.6-5.9) micrometers, contained eight sporozoites and a refractile residuum. The elongate form of A. saraviae n. sp. oocysts, and their more delicate walls, clearly distinguish them from oocysts of A. chagasi (Adler & Mayrink, 1961).

Animals↗
PubMedDate not supplied

Molecular phylogenetic relatedness of Frenkelia spp. (Protozoa, Apicomplexa) to Sarcocystis falcatula Stiles 1893: is the genus Sarcocystis paraphyletic?

The coccidians Frenkelia microti and F. glareoli (Apicomplexa: Sarcocystidae) form tissue cysts in the brain of small rodents (intermediate hosts) while oocysts are formed in the intestine of final hosts, buzzards of the genus Buteo. The inclusion of the small subunit ribosomal RNA gene sequences (SSU rRNA) of both Frenkelia species into the SSU rRNA trees of other, tissue cyst-forming coccidia strongly supports paraphyly of the genus Sarcocystis. Frenkelia spp. exhibit close relatedness to Sarcocystis falcatula Stiles 1893, a bird-opossum parasite, recognized under its junior synonym S. neurona Dubey et al. 1991, as the causative agent of equine protozoan myeloencephalitis on the American continent. As the definition of the genus Frenkelia is based on a plesiomorphic character (affinity to the neural tissue) of supposedly low phylogenetic value, the synonymization of the genus Frenkelia with Sarcocystis is proposed. This renders the genus Sarcocystis monophyletic.

Animals↗
PubMedDate not supplied

Directed migration of Ascogregarina taiwanensis (Apicomplexa: Lecudinidae) in its natural host Aedes albopictus (Diptera: Culicidae).

Directed migration of trophozoites from the midgut toward the Malpighian tubules is essential for Ascogregarina taiwanensis (Apicomplexa: Lecudinidae) to complete its developmental cycle within the natural host Aedes albopictus. We have obtained a 275-bp actin cDNA fragment amplified from extracted mRNAs of migrating trophozoites, suggesting the involvement of actin in trophozoite motility. Down-regulation on the migration of the trophozoite was seen in mosquito larvae fed with cytochalasin D, ML-7, and BDM, indicating that myosin, in the form of an actomyosin system, may also be involved in driving motility of the trophozoite. The "protruding apparatus" (PA) formed at the anterior end of trophozoites during the migrating stage had significant deposits of actin by immunofluorescent microscopy. Moreover, PA formation was enhanced in response to elevated levels of 20-hydroxyecdysone (20-HE) in cultures of alimentary canals in which the trophozite was contained. Thus, 20-HE may also promote expression of actin and perhaps myosin simultaneously.

Actins↗
PubMedDate not supplied

Serological affinities of the oyster pathogen Perkinsus marinus (Apicomplexa) with some dinoflagellates (Dinophyceae).

The protozoan oyster pathogen Perkinsus marinus is classified in the phylum Apicomplexa, although molecular-genetic and ultrastructural evidence increasingly concur on its closer phylogenetic relationship with the dinoflagellates. To test for evidence of serological epitopes common to P. marinus and dinoflagellates, we probed 19 free-living and 8 parasitic dinoflagellate, or dinoflagellate-like, species for cross-reactivity with polyclonal antibodies to P. marinus. Three of 19 free-living dinoflagellates (16%), and 7 of 8 parasitic dinoflagellates (88%) were labeled by anti-P. marinus antibodies. In reciprocal immunoassays using polyclonal antibodies to the Hematodinium sp. dinoflagellate parasite of Norway lobsters, Nephrops norvegicus, P. marinus and the same 7 parasitic dinoflagellates labeled by anti-P. marinus antibodies, were again labeled. The dinoflagellate-like parasite of prawns Pandalus platyceros was not labeled by either antibody reagent. These reciprocal results confirm the presence of shared antibody-binding epitopes on cells of P. marinus and several dinoflagellates. The apparent widespread serological affinity between P. marinus and the parasitic dinoflagellates suggests a closer phylogenetic link to the syndinean dinoflagellate lineage. The consistent failure of the dinoflagellate-like prawn parasite to bind either antibody reagent shows that this parasite is serologically distinct from both P. marinus and Hematodinium-species parasitic dinoflagellates.

Animals↗
PubMedDate not supplied

Nematopsis gigas n. sp. (Apicomplexa), a parasite of Nerita ascencionis (Gastropoda, Neritidae) from Brazil.

A new species of Nematopsis (Apicomplexa, Porosporidae) is described from the mantle tissues of the seawater gastropod, Nerita ascencionis (Neritidae), collected in the Atlantic North off the coast of "Fernando de Noronha" Island (3 degrees 47' 57'' S, 32 degrees 25' 12'' W) situated about 350 km from the northeast coast of Brazil. Numerous oocysts, each contained in a parasitophorous vacuole, were found in the cytoplasm of phagocytes in the mantle tissue of the host. The phagocytes were surrounded by a thin wall composed of lucent material. The phagocyte cytoplasm contained a nucleus surrounded by numerous vesicles and some dense masses. The oocysts were 21.9 +/- 0.5 microm long, and 11.5 +/- 0.6 microm wide. The oocyst wall was 0.18-0.25 microm thick, and the apical zone contained a micropyle, 1.0-1.2 microm in diameter, covered by a canopy-like operculum about 0.25 microm thick. Externally, the oocyst wall was surrounded by numerous anastomosing microfibrils attached to the wall and extending towards the periphery of the parasitophorous vacuole. Some microfibrils formed a dense complex network that surrounded the oocyst in the middle of the parasitophorous vacuole, which opened only at the apical zone near the external region of the opercular system. On the basis of the data obtained by light and transmission electron microscopy and host specificity, the gregarine Nematopsis gigas is distinguished from the nearest species as a new species. The taxonomic affinities and morphological comparisons with other similar species of the same genus are discussed.

Animals↗
PubMed2005

A new Apicomplexa-specific protein kinase family: multiple members in Plasmodium falciparum, all with an export signature.

BACKGROUND: Malaria caused by protozoan parasites of the genus Plasmodium spp. is a major health burden in tropical countries. The development of new control tools, including vaccines and drugs, is urgently needed. The availability of genome sequences from several malaria parasite species provides a basis on which to identify new potential intervention targets. Database mining for orthologs to the Plasmodium falciparum trophozoite protein R45, a vaccine candidate, led us identify a new gene family. RESULTS: Orthologs to the P. falciparum trophozoite protein R45 were detected exclusively in protozoan parasites of the phylum Apicomplexa, including several Plasmodium spp., Toxoplasma gondii and Cryptosporidium parvum. All family members are hybrid genes with a conserved C-terminal protein kinase domain of a novel type, recently called FIKK kinase, associated with a non conserved N-terminal region without any known functional signature. While a single copy gene was detected in most species, considerable gene expansion was observed in P. falciparum and its closest phylogenic relative P. reichenowi, with 20 and six copies, respectively, each with a distinct N-terminal domain. Based on full length protein sequence, pairs of orthologs were observed in closely related species, such as P. berghei and P.y. yoelii, P. vivax and P. knowlesi, or P. reichenowi and P. falciparum. All 20 P. falciparum paralogs possess a canonical Plasmodium export element downstream of a signal / anchor sequence required for exportation outside the parasitophorous vacuole. This is consistent with the reported association of the trophozoite protein R45, the only paralog characterised to date, with the infected red blood cell membrane. Interestingly, most genes are located in the subtelomeric region of chromosomes, in association with other multigene families contributing to the remodelling of the infected red blood cell membrane, in particular the ring erythrocyte surface antigen multigene family. CONCLUSION: This Apicomplexan-specific gene family was called R45-FIKK kinase. The family hallmark is a kinase domain with unusual characteristics, raising the possibility of designing drug or vaccine strategies targeting this domain. The characteristics of the P. falciparum family suggest a role in remodelling the infected cell and as such possibly contribute to the particular virulence of this species.

Animals↗